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power switches off

Posted by superdonkey 
power switches off
February 13, 2017 06:13AM

I have a problem with my new printer. A little bit after I start a print something kills the steppers and heaters. I have no clue what is the cause of this, I already swapped the arduino+ramps and even the powersupply, but without result. On the LCD nothing happens except for the temperature to drop. I also checked the resistance of the steppers, but nothing strange there. Any ideas what could be wrong or what I could try?

Re: power switches off
February 20, 2017 12:20AM
It might be that your resettable fuses are going off. They are F1 and F2 on your RAMPS board. Big yellow flat things (on mine) near the power connector.
From what I can tell, they aren't really fuses, but thermistors, PTCs. So if they get hot, they go high resistance and limit the current, dropping the output voltage.
If the one supplying your hot end goes high resistance, it will cut off power to the hot end and it will cool. I think what happens then is that when the temperature drops below some threshold, Marlin stops the print and everything stops, it is waiting for the temperature to come back up. This would explain why the steppers appear to be dead.
The FET on the RAMPS board is still on and trying to apply current to the hotted, so some small amount of current still flows through the 'fuse' keeping it hot.
One experiment you can try is to manually turn the hot-end temperature down via the LCD for a few minutes (stopping the small amount of current flowing through the 'fuse', giving it a chance to cool), then setting it back to where it should be to see if the temperature comes back up. The problem will still be present, but this will help to understand what is happening.

So why do the fuses get hot?
The obvious reason they get hot is if too much current flows through them. What is the resistance of the hot end resistor/cartridge? The fuse for the hot end is 5A rated. (Note that it powers other things as well as the hot end).
The other reason they can get hot is conducted heat and ambient heat. Make sure they are standing freely and not pressed against each other of other components so they can naturally cool.
Check that the power supply connectors are in good condition and making good contact. If they are not then they will generate heat and that will mess with the fuses. I know this can happen because it happened to me!! The power supply connectors on my RAMPS got a bit overcooked over time (mostly due to maintaining the bed at high temperatures for ABS) and the connections deteriorated. The heat from the connectors conducted to the nearby fuses and they would go high resistance.

So do your test with the hot end temperature control I mentioned above. Also do the same with the heat bed temperature (it draws a lot of current and will generate a bit of heat at a bad connection). Do a visual inspection on your connectors and solder joints, etc.

Let us know how you go.
Re: power switches off
February 21, 2017 09:30AM
Are you printing over USB? Perhaps your USB port is going to sleep? Try printing from the SD card.
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